Objects in the sky  the Sun, Moon, and stars, and their
positions as viewed from the Earth  helped early time-keepers
mark the passing of time. Ancient civilizations used the cycles
of these celestial objects to keep track of the seasons and to
produce early calendars. Early people used the apparent position
of the Sun, and later its shadow through the use of sundials,
to determine the time of day.

Clocks are a relatively new idea and were developed to help people
determine the time, both day and night. All clocks need two components:
something to produce regular cycles and something to keep track
of and display the time. The earliest clocks used flowing or dripping
water. Later in Europe, in about 1400, weight-driven pendulum
clocks were first used. About 100 years later, the first spring
driven clocks appeared, making portable timepieces possible, but
these early clocks were quite inaccurate. Early clocks used a
swinging pendulum or an oscillating wheel to tick off the seconds
(see the animation above).

In order to improve time-keeping, something was needed that had
a cycle of less than a second; as a result, clocks using miniature
tuning forks and quartz crystals were developed in the 1920s.
In the 1950s a clock that used the vibration of an atom was developed.
These Atomic clocks use the rapid vibration of the Cesium atom
to provide an accuracy of billionths of a second per year.

To find out more about time zones, clocks, calendars, and the
current time, go to:http://www.time.gov/